Image recording sheet



United States Patent Office 3,533,788 Patented Oct. 13, 1970 3,533,788 IMAGE RECORDING SHEET Robert F. Coles, North St. Paul, Louis A. Errede, Roseville Village, and Richard A. Miller and Horatio S. Stillo, White Bear Lake, Minn., assignors to Minnesota Mining and Manufacturing Company, St. Paul, Minn., a corporation of Delaware No Drawing. Filed Aug. 25, 1967, Ser. No. 663,247

Int. Cl. G03g 5/06 U.S. Cl. 961.5 14 Claims ABSTRACT OF THE DISCLOSURE This invention describes a variety of different constructions containing one or more semiconductive layers, the semiconductive layers containing novel organic semiconductor systems. These semiconductor layers are also photochromic and photoconductive. Their use is suggested in image recording sheets, light measuring devices and the like, where semiconductor properties and particularly photochromic and photoconductive properties are desired.

This invention relates to articles of manufacture having one or more organic semiconductor layers. In one aspect this invention relates to novel photoconductive constructions in the form of sheets or films. Still another aspect relates to novel photochromic layers.

'Photochromism and photoconductivity are well known phenomena used to advantage in many ways. Photoconductive ecording elements are important in the field of electrostatic electrophotography, as described in U.S. 3,051,569, and other photoconductive recording elements are useful in the field of electrolytic electrophotography, as illustrated by U.S. 3,010,884 and 3,252,874. Dye sensitization of photoconductive zinc oxide copysheets has been used to improve their photoresponse characteristics, as disclosed in U.S. 3,052,540. Most of such photoconductive recording elements have incorporated one or more photoconductive inorganic compounds, such as zinc oxides, cadmium sulfide, indium oxide, etc. For many imaging processes it is desirable to obtain an image transparency of the original light image, and processes have been devised to transfer image forming materials from the surface of the opaque, inorganic photoconductor sheets to a transparent receptor sheet. In recent years a number of organic photoconductive materials, including anthraquinones (U.S. 2,297,691; 2,663,636), have been reported in the literature, but their practicality for image recording has generally been limited by their relatively low sensitivity and response characteristics when exposed to a light image. Photochromic materials have also been suggested for use in image recording. However, photochromic materials which are organic photoconductors are not believed to be known heretofore.

It is an object of this invention to provide novel articles of manufacture having one or more layers containing organic semiconductors.

Still another object of this invention is to provide sheets and films having one or more novel photoconductive and photochromic layers.

A further object of this invention is to provide radiation sensitive organic materials in the form of continuous, uniform layers.

Yet another object of the invention is to provide novel image recording media.

Other objects and advantages will be apparent from the following disclosure.

The new and useful articles of manufacture of this invention are characterized by at least one continuous, uniform layer comprising at least one l-carboxamidoanthraquinone in which the amino nitrogen atom is directly bonded to the anthraquinone nucleus and has one hydrogen atom bonded directly thereto and at least an equimolar amount of basic amine compound having a pK, value no greater than benzylamine, said l-carboxamidoanthraquinone and said basic amine compound together having an electron spin resonance value (as hereinafter defined) of at least one when measured in solution in an inert solvent, e.g. dichloromethane, at 0.01 molar concentration of said 1-carboxamidoanthraquinone and 0.01 molar concentration of said basic amine compound. Illustrative basic amines which are useful in the photoconducfive layers include triethylamine, piperidine, diethyl amine, secondary butyl amine, n-butyl amine, isobutyl amine or benzyl amine. Many pK values for other basic amines appear in the reported literature. The l-carboxamidoanthraquinone may contain basic amine substituents and therefore may itself serve as the required basic amine compound, thus eliminating the need for a separate basic amine. In one embodiment a basic amine substituent group is attached to the carbon of the l-carboxamido rad ical in the l-carboxamidoanthraquinone and this basic amine substituent preferably contains only secondary or tertiary amine groups to enhance stability over an extended period of time and to minimize unwanted side reactions, e.g. condensation reactions. Such l-carboxamidoanthraquinones with basic amine substituents have been used as developer dyes in electrolytic electrophotographic imaging processes, e.g. see Example E in U.S. 3,172,827. A basic amine group may also be attached to the l-carboxamidoanthraquinone other than through the l-carboxamido radical, but it is generally desirable to avoid direct attachment of the basic amine substituent to the anthraquinone nucleus by using an intervening or bridging radical. However, whether the 1-carboxamidoanthraquinone is itself the basic amine compound or whether a separate basic amine compound is used together with the l-carboxamidoanthraquinone, it is essential that the maximum pK and minimum electron spin resonance requirements set forth above be followed. One or more of each constituent, i.e. the l-carboxamidoanthraquinone and the basic amine compound, may be incorporated into the layer. As used herein, reference to l-carboxamidoanthraquinone and basic amine compound is therefore intended to include both a single compound meetings both requirements as well as mixtures of one or more 1- carboxamidoanthraquinones and one or more basic amine compounds. It is also to be understood that the term l-carboxamidoanthraquinone includes compounds having other substituents on the anthraquinone nucleus. By uniform layer is meant a layer which has uniform composition and thickness. By continuous layer is meant a layer which is essentially uninterrupted over substantially the entire useful area of the construction in which it is a part, and the layer is therefore not selectively disrupted to form an imagewise pattern, i.e. image-free.

To compare the pK of the basic amine compound to that of benzyl amine it is convenient to measure the K, value in water at 25 C. when the basic amine compound is sufficiently water soluble. As measured under such conditions the pK, of the basic amine compound should not exceed a value of about 5. If the basic amine compound is insufliciently water soluble the pK value may be measured in another solvent, such as ethyl alcohol, and compared against the K of benzyl amine in that solvent. Comparative pK measurements should always be made in a solvent capable of dissolving both the basic amine compound and benzyl amine.

The above defined continuous uniform layers may contain the constituents in the form of discrete particles, either with or without a matrix or medium serving as binder. For example, the constituents may be deposited onto a suitable substrate as a powder or may be dispersed in a polymeric binder and coated onto a substrate or cast as a self-supporting layer to form a film or sheet. The continuous uniform layers may also contain the constituents in solution. The solvent medium may be liquid 'but is preferably a normally solid film forming composition which can serve as a binder or carrier. Polymeric binders are particularly useful, and weight percent or more (usually at least 30 weight percent or more (usually at least 30 weight percent) of the photoconductive layer is normally a polymeric binder in which the 1- carboxamidoanthraquinone and basic amine compound are either dissolved or dispersed, preferably the former. When light sensitivity is required, the binders used may be light transmissive and preferably transparent. If desired, additional organic or inorganic photoconductive materials, inorganic halides that form color centers (e.g. potassium bromide), sensitizers (e.g. eosin, erythrosin, etc.), fillers, plasticizers and pigments and the like may also be included in the layer, depending on its ultimate use.

Irradiation of the above photoconductive layers with light, such as light of wavelength below 5600 angstroms, is believed to generate relatively stable free radicals detectable by electron spin resonance and produce a rapid increase in electrical conductivity. These layers are also photochromic, and it has been found that the observed color change upon light exposure may be reversed by the addition of paramagnetic compounds, such as oxygen, nitrous oxide, and cupric ion. Accordingly, the photosensitive layers of this invention should be protected from exygen, such as by an oxygen impermable barrier layer or coating, unless image erasure or a relatively shortlived effect is desired.

Although some quinones and anthraquinones are known to have photoconductive properties, their level of photoconductivity is quite low and their response characteristics (e.g. rise time) are poor. By contrast, the photoresponse characteristics of the photoconductive layers of this invention include a rapid rise time (i.e. rate of increase in conductivity upon light exposure) and good photosensitivity, with a ratio of light conductivity to dark conductivity of at least 50 being quite common under the measurement test conditions hereinafter set forth. This is shown in Table I, which presents a variety of 1- car boxamidoanthraquinone and other compounds, and

Table II, which sets forth pK photoconductivity measurements, and electron spin resonance of samples prepared with these compounds. Coating mixtures were made by ball milling the indicated ingredients in a 10 weight percent solution in 1,2-dichloroethane of Butvar B-76 (a polymer of 45,00055,000 molecular weight having 9.0-l3.0 percent polyvinyl alcohol, 2.5% polyvinyl acetate, and -88 percent polyvinyl butyral, supplied by Shawinigan Resins Company., Springfield, Mass), a 10 weight percent solution in toluene of Pliolite 8-7 (a copolymer of weight percent styrene and 30 weight percent butadiene), or in approximately a /2 percent solution in chloroform without a polymeric binder. Ball milling was continued for at least 4 hours or until a uniform dispersion or complete solution was obtained. The coating mixture was knife coated onto the vapor coated aluminum surface of a polyester sheet to provide a coating thickness of about 0.5 mil after air drying under ambient conditions in a dark room for at least 24 hours. Bulk photoconductivity measurements were made in a cell in which a transparent conductive electrode was brought into contact with the coated samples, using a DC voltage of either 15 or 40 volts, a pressure of 10 pounds per 1.45 cm. across the cell electrodes, and a nitrogen atmosphere. The transparent electrode was connected as the cathode in all samples except samples 5 and 30-32, in which the transparent electrode was the anode. The sample strips (1 inch x 5 inches) were placed in the cell under red safe light and were exposed to white light from a 500 watt filament projection lamp. Light intensity at the sample surface was adjusted to about 700 foot-candles. The current passing through the cell was calculated from voltage drop measurements across a load resistor, and the current flow in the dark and under light exposure was converted to dark conductivity (0 and light conductivity (11 respectively.

Table III reports electron spin resonance (ESR) results of l-benzoylamido anthraquinone with various amines. It can be seen that the amines with pK values no greater than benzylamine have electron spin resonance values of at least about 1 and are therefore useful basic amines in the light sensitive layers of this invention. Furthermore, a molar excess of basic amine over the molar amount of the 1-carboxamidoanthraquinone, may increase the electron spin resonance, as shown in Table III.

TABLE II-Continued Photoconductive response Other in- Photoconductor gredients, Light conduc- Rise time in min- ES R peak height, Sample system (parts by pk Binder (parts by (parts by tivity, a1, (ohmutes (time to reach cm. in CHzClz weight in brackets) weight in brackets) Weight in cm.)- (applied indicated multiple (IL/'1) test solution (molar brackets) D.C. voltage of dark conductivity cone. in brackets) in brackets) in brackets) 26 Compound IV [1. 0].- 4. 3 ButvarB-76 [1. 23] KBr [3.0] 2. 41 10 [15] 0. 02 [100] 793 1. 0 27 Compound 1V [1. 0].. 4. 3 Butvar B-76 [4. 92] 1. 30 10- 15 1. 42 1. 0 28 Compound IV [2. 0] 4. 3 Butvar B-76 [1. 23] ZnO [3.0] 2. l l0- 0. 24 [10, 000]. 693,000 1. 0 29 Compound 4. 3 None 3. 8l 10 [15]. 0. 44 [1, 000] 1,160 1. 0 30 Compound IV 4. 3 None 6. 94 10- [15] 0. 13 [1, 000] 3, 710 1. 0 31 Compound IX 5. 6 None 1. 68 10 [15] 0. 43 [10] 12 32 Compound XXIIL- 10. 0 None 1. 51 10- [60]..- Rise time too slow 1. 00 1. 0

0 measure.

TABLE III.ELECTRON SPIN RESONANCE RESULTS IN METHYLENECHLORIDE SOLUTION 1,5 diacetyl amide l-benzoyl amido anthraquinone (BAQ) anthraquinone (DAQ) Concentration, Concentration, Concentration, Concentration,

Amine, moles/liter Peak moles/liter Peak moles/liter Peak moles/liter Peak pKe in eight, height, height, height,

Amine water BAQ Amine cm. BAQ, Amine cm. BAQ, Amine cm. DAQ, Amine cm.

Piperidine 2. 80 0. 012 0. 5 127. 7 0049 2. 0 Diethylamine..- 2. 90 0. 012 0. 5 28. 4 0049 2. 0 Trlethylamine- 3. 25 0. 012 0. 5 1, 872 0049 2. 0 n-Butylamine- 3. 39 0. 012 0. 5 3. 6 0049 2. 0 Sec.-Butylamine..- 3. 44 0. 012 0. 5 4. 75 0049 2. 0 Iso-Butylamine- 3. 59 0. 012 0. 5 1. 40 0049 2. 0 Benzylamine- 4. 70 0. 012 0. 5 -1 0049 2. 0 7. 05 0. 012 0. 5 0 0049 2. 0 8. 85 0. 012 0. 5 Trace 0049 2. 0 I 8. 1 0. 012 0. 5 O 0049 2. 0 9. 20 0. 012 0. 5 0 0049 2. 0 Glycine 11.65 0. 012 0. 5 75 0049 2.0

The electron spin resonance (ESR) data were obtained by preparing solutions of the anthraquinone and basic amine at the indicated concentrations in dichloromethane, an inert solvent which has essentially no measurable ESR value, adding 0.5 m1. of the solutions to quartz tubes having an internal diameter of 3 mm., degassing the samples under vacuum conditions, and sealing the tubes under vacuum while immersed in liquid nitrogen. Electron spin resonance determinations were made in a dual cavity apparatus with peroxylamine disulfonate as reference, the instrument parameters being 8 gauss modulation amplitude, 0.3 second time constant, 9.3)(10 cycles/second frequency, 0.9 inch x 0.4 inch cavity aperture, and 50% light attenuation by aperture screen. Since the instrument parameters are not suflicient to specify apparatus sensitivity, diphenylpicrylhydrazyl was used as an internal standard in the following manner. The exact concentration of an approximately 0.04 molar solution of diphenylpicrylhydrazyl in dichloromethane was determined by spectrophotometric measurement. The molecular extinction coefficient of diphenylpicrylhydrazyl in chloroform at 530 m is reported as 37 (g./l.) (cm)- in J. Chem. Physics, 36, 1197-1208 (1962), and the absorption spectrum in dichloromethane was almost identical to that in chloroform, which permits the use of the literature data for the analysis. Electron spin resonance measurements of the internal standard were made periodically by substituting diphenylpicrylhydrazyl solution for the sample. The standard solution is not illuminated during measurement. The peak height of the standard signal is adjusted to 0.04 molar diphenylpicrylhydrazyl by multiplying the observed signal by the ratio 0.04/exact molar concentration of the standard solution. The sensitivity of the apparatus is specified by the statement: peak height of 0.04 M diphenylpicrylhydrazyl:16600 x peak height of minimum electron spin resonance photosignal of sample. Measurement of the photoinduced electron spin resonance signal of a sample is first made in the dark. The sample was then exposed in the cavity to unfiltered white light from a 500 watt projection lamp placed 12 inches from the aperture screen, providing 13,000 foot-candles of incident light on the sample. Since the photo-induced signal of interest has a g value of 2.005, this spectral region is repetitively scanned to monitor the signal growth during illumination over a period of four minutes, at the end of which the signal intensity was recorded. In general, a symmetrical ESR peak (g=2.005) with a line width of about 5 gauss is formed, and the peak height (i.e. distance between the maximum and minimum value of the first derivative signal) is readily measured.

The properties displayed by the continuous uniform layers of this invention suggest a variety of uses. As organic semiconductors they may be employed in semiconductor devices and systems. Their photoconductivity makes them particularly useful in image recording elements and sheets for electrophotographic processes, in light metering and light measuring devices, etc. Because of their photochromic properties, such layers may be incorporated into sheets and films which then can be imaged directly upon light exposure, and the resultant prints may be rendered permanent by protecting the image layer from contact with oxygen. Photochromism has many other applications, such as in sunglasses, light modulation, light filtration, etc. In addition to their sensitivity to electromagnetic irradiation these layers may also be directly imaged by exposure to other forms of irradiation, such as an electron beam. Due to the unique properties and useful characteristics of the continuous uniform layers of this invention it will be apparent that the manufactured article in which they are contained can be extremely varied in form and in content, depending on the ultimate use of the product.

The following examples will illustrate particular uses for the constructions of this invention.

EXAMPLE 1 A mixture of 4 parts by weight of Compound 1V in Table I and 1 part by weight of styrene/butadiene copolymer (70/30 weight percent respectively) was coated onto the polyester side of an aluminum coated 5 mil polyester sheet. This uniform, continuous coating (0.5 mil dry thickness) was then overcoated with polyvinyl alcohol to produce a film laminate of about 6 mils total thickness. Samples of this film were exposed to 17,000 foot-candle-minutes of visible light through a suitable negative with the production of a positive black image having an optical density of 1.60 as contrasted with a background optical density of 0.50. Although the image was permanent as long as the polyvinyl alcohol topcoat 13 remained intact to protect the image from oxygen, image erasure occurred within minutes when the polyvinyl alcohol topcoating was manually separated from the laminate. This provides a construction which, though capable of forming and preserving an image, can. be erased if desired, to permit reuse of the sheet.

EXAMPLE 2 A mixture of 4 parts of the l-amidoanthraquinone of Example 1 and 1 part of Butvar B-76 was coated onto a 5 mil polyester sheet, and a polyvinyl alcohol topcoat was then applied. Samples of this film were exposed to 9,500 foot-candle-minutes of visible light through a suitable negative to give a black image with optical density of 1.2 and a background of 0.42. Using this sheet as a graphic original additional copies were prepared in a commercial thermographic copying apparatus. The image on the original sheet was then erased by warming the film to 95 C. and maintaining this temperature for about 8 minutes. The clean sheet was then re-exposed at room temperature through a second negative to produce a new image that was again used as a graphic original to provide 10 dry copies in a commercial thermographic copying apparatus.

EXAMPLE 3 A mixture of 4 parts by weight of l-amidoanthraquinone of Example 1 and 1 part by weight of styrene/ butadiene copolymer (70/30 weight percent respectively) was knife-coated onto 2 mil aluminum foil and dried. A sample of this coated film (0.5 mil thickness) was exposed at 1.6 milliseconds/inch to a 20 kev. electron beam. A visible dark image formed at 25 microamperes electron beam current. Exposure to atmospheric oxygen caused the visible image to fade.

A solution of 3 parts 'by weight of Compound I from Table I and 1 part of Butvar B-7-6 in 1,2-dichloroethylene was coated. onto 1 mil polyester and dried. The coated side was then treated with an acrylate resin latex to provide a 0.5 mil transparent, protective topcoating. The dry film was then exposed for 1 minute to white light of 38,- 000 foot-candles intensity to cause a noticeable color change from amber to brown, due to the 'broad absorption band of the l-amidoanthraquinone photoproduct in the 500-700 millimicron wavelength range. The transmittance of 550 millimicron light, to which the dye is relatively insensitive was not significantly changed. The original absorption spectrum was restored about 8 minutes after illumination was discontinued. The rate of response and rate of restoration is a function of the topcoat material and is a function of the oxygen permeability of the topcoat.

EXAMPLE 4 A mixture of 4 parts by weight of the l-amidoanthraquinone of Example 1 and 1 part by weight of Butvar B-76 was coated onto a polyester film and dried. This film was then cut into strips, and the strips were placed in vacuum sealed containers, such as glass vials. These containers simulated packages for special foods and medicinals that are extremely sensitive to air. The coated strips in the transparent sealed containers were illuminated with ordinary light to change the yellow co or of the coating to blue. In the vacuum environment the blue color persisted. After several days the seals were deliberately broken on the glass vials, and the color of the strips immediately changes from blue to yellow because of the presence of oxygen. This illustrates the practical use of such layers and films containing such layers as an oxygen indicating system. Since the magnitude of color change is a function of both light exposure and available oxygen, the magnitude of change realized for a given light exposure is a quantitative measure of the amount of oxygen present in a system. This provides a non-destructive visual method for monitoring oxygen leakage of vacuum sealed, transparent containers.

Various other embodiments of the present invention will be apparent to those skilled in the art without departing from the scope thereof.

The embodiments of the inevntion in which an exclusive property or privilege is claimed are defined as follows:

1. An image recording sheet having at least one continuous, uniform photoconductive layer comprising at least one l-carboxamidoanthraquinone in which the amido nitrogen atom is directly bonded to an anthraquinone nucleus and has a hydrogen atom bondeed directly thereto and at least an equimolar amount of at least one basic amine compound having a pK value no greater than benzylamine, said amidoanthraquinone and said basic amine compound together having an electron spin resonance value of at least one when measured in solution in an inert solvent at 0.01 molar concentration of said l-carboxamidodoanthraquinone and 0.01 molar concentration of said basic amine compound, the concentration of said l-carboxamidoanthraquinone and said basic amine compound being suflicient to provide a minimum ratio of light conductivity to dark conductivity of 50.

2. The image recording sheet of claim 1 in which said continuous, uniform layer contains a polymeric binder.

3. The image recording sheet of claim 2 in which said polymeric binder is a light transmissive, normally solid polymer.

4. The image recording sheet of claim 1 in which said l-carboxamidoanthraquinone and said basic amine compound are the same chemical compound.

5. The image recording sheet of claim 4 in which said l-carboxamidoanthraquinone contains a 'basic amine substituent attached to the anthraquinone nucleus through the amido group.

6. The image recording sheet of claim 1 in which said 1-carboxamidoanthraquinone and said basic amine compound are separate chemical compounds.

7. The image recording sheet of claim 1 in which said layer is protected from contact with oxygen.

8. The image recording sheet of claim 7 in which said layer is protected from contact with oxygen by an oxygen impermeable barrier layer.

9. The image recording sheet of claim 1 in which said layer comprises a solution of said l-carboxamidoanthraquinone and said basic amine compound in a normally solid, film forming binder.

10. The image recording sheet of claim 1 in which said layer comprises a dispersion of said l-carboxamidoanthraquinone and said basic amine compound in a normally solid, film forming binder.

11. The image recording sheet of claim 1 in which said construction consists solely of said layer in self-supporting form.

12. An image recording sheet having at least one continuous, uniform, photoconductive layer comprising a binder and at least 1 part by weight per 2.23 parts by weight of total solids of at least one l-carboxamidoanthraquinone in which the amido nitrogen atom has a hydrogen atom bonded directly thereto and at least an equimolar amount of at least one basic amine compound having a pK value no greater than benzylamine, said amidoanthraquinone and said basic amine compound together having an electron spin resonance value of at least one when measured in solution in an inert solvent at 0.01 molar concentration of said l-carboxamidoanthraquinone and 0.01 molar concentration of said basic amine compound, the concentration of said 1-carboxamidoanthraquinone and said basic amine compound being suflicient to providea minimum ratio of light conductivity to dark conductivity of 50.

13. The sheet of claim 12 in which said image recording sheet consists only of said continuous uniform layer in self-supporting form.

14. The sheet of claim 12 in which said image recording sheet comprises a support and at least one continuous uniform layer as therein defined.

References Cited UNITED STATES PATENTS 1 6 3,305,361 2/1967 Gaynor et a1 9689 3,436,401 4/1969 Pfister 260377 X 3,439,003 4/1969 Reich et a1. 260377 GEORGE F. LESMES, Primary Examiner C. E. VAN HORN, Assistant Examiner US. Cl. X.R.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,533,7 Dated October 13, 1970 Robert F. Coles Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 63, "pK" should read --pK Column 3, lines 9 and 10, "(usually at least 30 weight percent or more (usually at least 30 weight percent)" should read --(usually at least 30 weight percent)-. Column 3, line 32 "exygen should read oxygenand "impermable" should read -1mpermeable-. Table I No. I should read O NHCO Table I, No. XVI, should read NHCO " CONH-(CH --N(CI1I CH CH CH 0 Table I, No. XVII should read o NHCO-QCONH-(CH -u-(cH -N(cH I CH -CH -CH(CH2) on O 3 2 Table I, No. XVIII, should read Signed and sealed this 6th day of April 1971.

(SEAL) Attest:

EDWARD MELETCHERJR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents FORM F'O-IOSO (10-69) USCOMM-DC 6O375-P69 us GOVERNMENT vnnmuc orrace; I!" o-ast-su 

